1. Field of the Invention
The present invention relates to a method for treating impure water, and more particularly to an improved method for treating sewage to produce treated effluent of very high quality.
2. Description of the Background Art
It is known that color, turbidity, organic matter and similar impurities may be removed from water by coagulants, e.g. alum, ferric sulfate or the like. These compounds are acidic and react with the alkalinity in the water or with alkaline compounds, e.g. lime or soda ash, to form voluminous insoluble precipitates (hydrates). The precipitates have a tremendous surface area on which the dissolved or colloidally dispersed impurities are absorbed. The suspended impurities are surrounded by the gelatinous hydrates and become part of the precipitate.
Domestic or sanitary sewage and industrial wastes may be purified by the chemical precipitation process, in which suitable chemicals (e.g. aluminum sulfate, lime, iron chloride, polyelectrolytes or combinations thereof) are added to the sewage and the sewage passed to one or more flocculating tanks, normally equipped with slowly rotating agitators or paddles, in which colloidal solids are formed into particles of size and weight that will settle. The colloidal solids or flocs are then separated from the liquid by being allowed to settle in subsequent settling tanks, whereafter the purified water is collected in a weir structure mounted at the surface of the water, while the sediment, consisting of flocs and sludge, is removed, normally by means of sludge scrapers and/or pumps.
The prior art teaches the addition of various types of chemicals and combinations of chemicals to sewage and other impure water to remove various pollutants therefrom.
There are several deficiencies in all of the prior art which the present invention overcomes as indicated below:
1. The invention, when used to treat raw sewage or other impure water with very economical doses of three chemicals converts a very high proportion of the suspended, colloidal and dissolved pollutants in the sewage or other impure water to large, dense and stable flocs which are so resistant to shear forces they can be settled out in a clarifier without the aid of inclined sedimentation means, and with an upward flow velocity of at least eighteen to twenty meters per hour. This flow rate is approximately ten times higher than recommended by those skilled in the art for clarifiers without inclined sedimentation means. (Ref. p. 149 - Nutrient Control--Manual of Practice FD-7-Facilities Design by the Water Pollution Control Federation 1983.
The foregoing is a very important advantage from an economic point of view because it allows the use of a very much smaller clarifier, and reduces the area of land required for a treatment plant.
2. The invention, notwithstanding the fact that very economical doses of chemicals are used and the floc is settled against an upward velocity flowrate of 18-20 m/hr. without inclined sedimentation means, achieves removal rates of pollutants which heretofore have not been possible as indicated hereunder: PA1 3. The invention is a considerable improvement over the prior art in relation to the removal of Biochemical Oxygen Demand (BOD.sub.5), with approximately 95% of all BOD.sub.5 over 0.2 microns in size being removed, and in addition, almost one third of the BOD.sub.5 less than 0.2 microns in size also being removed. PA1 4. When sewage or other impure water is treated using the methods described in this invention, the percentage removal of suspended solids and turbidity is significantly greater than can be accomplished by the prior art taking into account the dosage of chemicals and the flow rates through the clarifier. PA1 5. A very important advantage of this invention over the prior art is its versatility. The invention can be used as either a Primary and/or Secondary and/or Tertiary Treatment system, and can be combined to advantage with other chemical, physical or biological processes. PA1 6. Another important advantage of this invention is the overall speed with which the treatment process takes place. While the overall retention time required is site specific and depends on such factors as the quality of the influent and/or the quality of the effluent required, typically, for sewage treatment the overall retention time is less than thirty minutes. PA1 7. The quality of the sludge produced by the use of this invention, while being site specific, is generally of a very high solids content and is readily thickened in a short period of time. The resulting thickened sludge is then readily dewatered to a high solids content cake. This is a very important aspect of this invention, and distinguishes this invention over the prior art in that the total volume of sludge to be disposed of is lower then usual, resulting in important economic and environmental advantages. PA1 (a) an inorganic coagulant, (b) an anionic polymer, and (c) a cationic polymer with intimate mixing of the added chemicals with the sewage or other impure water, with the proviso that (d) the inorganic coagulant either alone or with the anionic polymer or the cationic polymer cannot be added last; and (e) the anionic polymer and the cationic polymer cannot be intimately mixed and added together, thereby to provide chemically-treated effluent having large, compact, firmly-bonded, substantially shear resistant and rapidly separable flocs therein; separating the flocs from the liquid in a separating zone; and removing treated liquid effluent from the separating zone.
______________________________________ Pollutant Average % Removal ______________________________________ Biochemical Oxygen Demand (BOD.sub.5) 76% Dissolved BOD.sub.5 under 0.2 microns 32% in size BOD.sub.5 over 0.2 microns in size 95% Total phosphorus 97% Turbidity 95% Total Suspended Solids 92% Fats, Oils and Grease 90% Aluminum Removes all of the the aluminum which is dosed into the sewage or indus- trial effluent, in addition to approximately 70% of the small quantity of aluminum present in the influent. ______________________________________
The implications of this fact means that the invention can be used in many locations to treat raw sewage to a standard that does not require further treatment before discharge to waterways, whereas the effluent from other chemical systems requires additional biological treatment.
Furthermore, where highly polluting waste waters are treated in accordance with this invention and where the resulting treated effluent requires additional biological treatment, the pollutional load on the subsequent biological system is reduced to a significant extent, thereby resulting in substantial cost savings.
This is a very important improvement over the prior art, and eliminates the need for a subsequent filtration process in many instances.
It also allows the use of additional processes in many cases such as Ultraviolet Disinfection, Reverse Osmosis, Activated Carbon and/or Ammonia Removal using Clinoptilolite Ion Exchange Material without the use of an intervening filtration process.
Tests have indicated that raw sewage, after being treated using the methods described by this invention, and then passed directly through an ultraviolet disinfection apparatus, was efficiently disinfected and the resultant total coliform count was only 10 per 100 ml.
The system therefore easily lends itself to automation, which would have substantial economic advantages such as control of chemical dosages and reduction of labor costs.